AIf both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
a
Young'smodulus$(Y)=\frac{\text {stress}}{\text {strain}}$
If strain changes stress also changes since $Y$ is constant for a material.
Also, poisson's ratio $=\frac{\text { lateralstrain }}{\text { longitudinalstrain }}$
$\nu=0.5$ for rubber and is most elastic material.
So, both are correct but not reason of assertion.